A standard drum centrifuge has a generally cylindrical drum rotatable about and centered on an axis and having a foraminous outer wall with an inner surface, a tubularly cylindrical support screen lying directly on the inner surface, a tubularly cylindrical filter mesh lying directly on the support screen, and a tubularly cylindrical protective screen lying on the filter mesh and holding same against the support screen. The support screen holds the mesh off the drum surface so that the filtrate can collect under the mesh and run out drainage holes in the drum. The protective screen prevents a scraper blade of the type described in commonly owned patent application 07/479,711 filed Feb. 14, 1990 from directly contacting and potentially damaging the relatively fragile mesh.
It is essential to hold the support screen and mesh snugly in place both to prevent the suspension being filtered from getting past them and to keep the mesh flat so it works most efficiently. This retaining system at the same time must permit the filter mesh to be changed with relative ease as under the best of circumstances the filter mesh wears out. Finally the retaining system cannot get in the way of whatever system is used to strip filter cake from the inner surface of the mesh and/or protective screen, as in some systems a 10 mm to 15 mm thick residue is left that greatly reduces filtering efficiency.
In one known arrangement a system of tensioning frame and ring is held in place by a plurality of threaded connectors. The drum is formed with grooves into which the retaining ring fits. Such an arrangement is quite difficult to disassemble and reassemble when the filter mesh must be changed, and does not work in systems where the filter mesh is backflushed to clean it. Furthermore this arrangement is fairly complex and costly.
Another arrangement uses a dovetail-shaped groove in the drum in which the mesh edge is held by an appropriate ring. The drum here is fairly complex and expensive to manufacture, in particular because of the extra wall thickness necessitated by the grooving. In this arrangement also separate retaining devices are needed when the mesh is to be backflushed to keep into in place.